This invention relates to a linear array of continuous line hydrophones and, more particularly, to improved means for coupling continuous line hydrophone outputs to a data recording system.
In the field of seismic signal detection, it has long been recognized that a linear array of detectors having a length greater than one wavelength provides a directional response and thereby cancels undesired seismic signals. It has also long been recognized that a continuous line detector or hydrophone is the ultimate linear detector and that a hydrophone comprising a plurality of discrete detectors is at best only an approximation of such a continuous line detector. These facts were disclosed in U.S. Pat. No. 1,584,613, issued to D. F. Comstock, et al., for a Wave Detector, on May 11, 1926. The Comstock patent disclosed variable resistance, magnetic and electrostatic line hydrophones. The Comstock patent also disclosed in its last paragraph that a "self-activity" effect had been observed.
This self-activity effect is believed to be essentially the same as the response disclosed in U.S. Pat. No. 3,763,482, issued to C. F. Burney, et al., for a Coaxial Cable Transducer, on Oct. 2, 1973. In the Burney disclosure, a coaxial cable dielectric is permanently polarized by being heated and then cooled while being subjected to a high-voltage DC potential. As Burney discloses, this permanently polarized dielectric is commonly known as an "electret" and has been known since the early 1920s. Modern thermosetting plastics, which have long charge lives, have only recently made it practical to manufacture electret transducers.
Modern seismic prospecting systems typically include a plurality of seismometer groups or arrays arranged in a line or spread. In marine seismic prospecting systems, groups of hydrophones are assembled in a cable and the entire assembly forms a streamer which is towed behind a ship which carries recording equipment. Each group is large enough to provide the surface wave cancellation effect disclosed in the Comstock patent. The use of a large number of these groups allows the seismic inspection of a larger area, as well as providing many other improvements in the quality of data acquired. It has been common practice to interconnect the plurality of groups to recording equipment by means of a multiconductor cable running from the recorders to all of the groups. These cables have become quite large, complex, and expensive, as the number of groups has been increased and the surface coverage has been extended to the range of one mile. A U.S. Pat. No. 3,781,778, issued to Sawin, et al., on Dec. 25, 1973, for a Marine Streamer Cable, discloses a new configuration of a cable of this type.
Some of the problems associated with the use of multiconductor cables in seismic exploration were recognized in and solved by the U.S. Pat. No. 3,239,803, issued to J. J. Godbey, on Mar. 8, 1966, for a Variable Capacitance Geophone Assembly for Seismic Prospecting. Godbey's disclosure solves the multiconductor cable problems by providing a geophone which generates a carrier wave signal which is frequency-modulated by acoustic waves which impinge upon the geophones. Each geophone operates at a separate center frequency so that all the signals from a plurality of geophones may be coupled onto a single transmission line, such as coaxial cable. At the recorder end of the cable, electronic circuitry separates the signals according to frequency so that each geophone output is separately recorded, even though all are transmitted simultaneously over a single two-conductor cable.
Thus, it is seen that various continuous line geophones have been known and used and that methods of transmitting a plurality of geophone signals on a simple two-wire transmission line have been known, but that the advantages achieved by these two improvements have never been combined.
Accordingly, an object of the present invention is to provide an improved continuous line hydrophone.
Another object of the present invention is to provide a hydrophone streamer comprising linear transducer elements.
Yet another object of the present invention is to provide a hydrophone array in which all hydrophone outputs are coupled to a recorder by a single two-conductor transmission line.